1. Field of the Invention
The present invention relates to an inertial sensor.
2. Background Art
An inertial sensor manufactured by a machining technique of micro electro mechanical systems (MEMS) detects displacement of a movable part in order to measure an inertial force. The inertial sensor can measure an inertial force such as acceleration, an angular velocity, or angular acceleration by converting the displacement into an electric signal and processing the electric signal with an electronic circuit. In particular, a capacitance type inertial sensor detects the displacement as a change in the capacitance between a fixed detection electrode and a movable electrode. In the following explanation, a direction which is orthogonal to a principal plane of a substrate layer in which an elastic beam and a movable part are to be machined is referred to as an out-of-plane direction. For example, acceleration acting in the out-of-plane direction is referred to as out-of-plane acceleration.
In order to detect the acceleration in the out-of-plane direction, it is necessary to shift the center of gravity of the movable part from a rotation axis. An example of an inertial sensor having such a movable part is disclosed in JP-T-2010-536036 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application) (Patent Literature 1). In FIG. 1 of Patent Literature 1, lengths from the rotation axis to respective left and right ends of the movable part are set to different lengths. As a result, the center of gravity of the movable part is shifted from the rotation axis. On the other hand, for example, in FIG. 3 of Patent Literature 1, a part of the movable part is removed asymmetrically with respect to the rotation axis by etching to open a hole (the opened hole is referred to as “aperture”) to shift the center of gravity from the rotation axis while keeping the lengths from the rotation axis to the respective left and right ends of the movable part the same. In both the configurations of Patent Literature 1, an electrode is provided under the movable part. JP-T-2008-544243 (Patent Literature 2) is a literature similar to Patent Literature 1. For example, FIG. 3 discloses the configuration in which apart of the movable part is removed asymmetrically with respect to the rotation axis by etching while keeping the lengths from the rotation axis to the respective left and right ends of the movable part the same.
Another example of the acceleration sensor that detects acceleration in the out-of-plane direction is described in JP-A-2000-19198 (Patent Literature 3). Patent Literature 3 discloses a technique for providing a plurality of sub-supporting supports asymmetrically with respect to a rotation axis and suppressing an influence due to distortion of upper and lower substrates of a movable part. However, in Patent Literature 3, the movable part is configured asymmetrically with respect to the rotation axis. It is unclear how the center of gravity is shifted from the rotation axis.
As another related art, in recent years, a manufacturing process employing a transfer mold process for cost reduction in packaging an inertial sensor attracts attention. The transfer mold process is a manufacturing process explained below. First, an MEMS element, an LSI electronic circuit, and a lead frame are set in a mold and, then, warmed resin is filled in the mold at high pressure of about 5 to 20 MPa. The resin is cooled and solidifies to be a mold resin package for fixing the MEMS element, the LSI electronic circuit, and a leader line. The transfer mold process has higher mass productivity than a process employing a ceramic package in the past and is expected to be an effective process in reducing manufacturing costs for the inertial sensor.
In the MEMS element of the inertial sensor, the movable part is encapsulated in a cavity at the atmospheric pressure or in a vacuum. If the transfer mold process is applied to such an MEMS element, when the resin is filled in the mold at high pressure, the high pressure is applied to the MEMS element as well. Then, since a difference between internal and external pressures of the element is large, the cavity of the element is deformed. The coefficient of thermal expansion of a material (silicon, etc.) forming the MEMS element is different from the coefficient of thermal expansion of the resin. Therefore, the element is deformed when heat is absorbed and emitted in the transfer mold process. Further, the formed mold resin package has a characteristic to expand by absorbing heat and humidity and contract by emitting heat and drying. Therefore, the MEMS element is deformed depending on fluctuation in an environment in which the mold resin package is set.
As explained above, in the inertial sensor employing the transfer mold process, various factors of deformation are conceivable. Problems explained below occur because of the deformation.
Asymmetrical Distortion
First, a problem of asymmetrical distortion is explained.
When the configuration in which the lengths from the rotation axis to the ends of the movable part are changed (FIG. 1 of Patent Literature 1) and the configuration in which a part of the movable part is asymmetrically removed (FIG. 3 of Patent Literature 1) are compared, the inertial sensor having the former configuration has higher sensitivity under the same conditions. This is because, whereas, in the latter configuration, a portion where the aperture is formed in the movable part is a useless region not contributing to detection of an inertial amount, in the former configuration, the useless region is absent in the movable part. Further, in general, an effect of shifting the center of gravity is larger when the lengths to the ends of the movable part are changed to shift the center of gravity than when the aperture is formed to shift the center of gravity. Therefore, in the former configuration, it is possible to more largely change the position of the movable part having the same mass. As a result, the inertial amount can be detected at higher sensitivity.
A detection electrode opposed to the movable part is desirably provided on a substrate above the movable part. This is because a change is the capacitance in the MEMS element is extracted by wire bonding and transmitted to an LSI, when the detection electrode is provided above the movable part, it is easier to draw around the leader lines than when the detection electrode is provided below the movable part.
However, when a process for applying high pressure such as the transfer mold process is applied to an inertial sensor that satisfies such demands, asymmetrical distortion occurs in the detection electrode. The asymmetrical distortion is explained with reference to FIGS. 7 and 8. In FIG. 7, the lengths from the rotation axis to the ends of the movable part are set different on the left and right and the lengths from the rotation axis to cavity ends are also set different on the left and right. Detection electrodes 505a and 505b are respectively provided above a movable part 504.
When high pressure is applied to the MEMS element having such a configuration by the transfer mold process, the environment such as temperature and pressure fluctuates. Then, as shown in FIG. 8, upper substrates set in contact with the mold resin package show deformations different from each other. The detection electrodes 505a and 505b provided above the cavity respectively show different deformations. As a result, the capacitances between the detection electrodes and a movable electrode show changes different from each other. When the capacitances are detected by differential detection shown in FIG. 6, an offset occurs. This causes deterioration in sensor sensitivity. All the patent literatures neither describe nor indicate the problem of the asymmetrical distortion and means for solving the problem.
Distortion Due to High Pressure
Second, distortion due to high pressure is explained. In the transfer mold process, when the resin is filled, high pressure of about 5 to 20 MPa is applied to the MEMS element. In such a high-pressure process, a change in the detection electrode due to the distortion of the MEMS element is also large compared with the process in the past. Therefore, even if the inertial sensor is configured to be capable of reducing the influence of a relative change in the detection electrode due to the asymmetrical distortion, it is desirable that the inertial sensor can reduce an absolute change in the detection electrode as well. A support provided as in Patent Literature 3 is one of configurations for reducing such an absolute change. However, it is more suitable for the transfer mold process if the absolute change can be reduced by the configuration of the movable part as well. However, not only Patent Literature 3 but also all the patent literatures neither describe nor indicate such a configuration of the movable part.